The present invention relates to a transmission power control system in a digital satellite communication system, in particular, relates to such a transmission power control system which is used for a communication between small earth stations or micro-terminals.
A satellite communication system using a small earth station, or a micro-terminal, is promising due to the latest development of a large communication satellite, the latest development of digital communication system, and/or the latest development of a communication element. In particular, the Ku band (11/14 GHz) is preferable for future small earth stations satellite communication, in view of the low communication cost.
When the frequency higher than 10 GHz is used in a satellite communication system, the rain attenuation is a big problem in circuit management. In particular, when a small earth station which has a low elevation angle is used, that rain attenuation problem is serious and must be solved.
Conventionally, the solution for rain attenuation is space diversity system, or transmission power control system.
In a space diversity system, a pair of spaced two antennas is installed so that received signals at both antennas are not affected by rain simultaneously, and the antenna which is less affected by rain is selected for an actual circuit.
In a conventional transmission power control system, a transmission power of a transmitter is controlled based upon a level of reception signal, such as a beacon signal.
The former space diversity system becomes very costly due to the duplex system. It should be noted that a small earth station communication system must provide cheap communication.
The latter transmission power control system of a prior art has the following disadvantages.
(a) A transmitter installed in a small earth station has usually not enough surplus power for power control. Therefore, the range of the control is small.
(b) Several hundred small earth stations share a common transponder on a satellite, the transmission power for each channel is very small, and therefore, a circuit is damaged by rain attenuation of a downward circuit.
Accordingly, a conventional technique for solution of rain attenuation can not be applied to a small earth station communication, directly.